Gas separation



L. W. ROYER GAS SEPARATION Oct. 21, 1952 Filed Deo. 4, 1946 nventorCittotna'g 2238 wzubwa .wMVl

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LESLIE lll/.ROYER Bg l l huls Patented Oct. 21, 1952 p p GAS SEPARATIONLeslie W. Royer, Aliquippa, Pa., assigner to Koppers Company, Inc.,Pittsburgh, Pa., a corporation of Delaware Application December 4, 1946,Serial No. '713,899

This invention relates to the separation oi hydrogen sulphide from gasmixtures by selective absorption. More particularly the inventionrelates to the separation of hydrogen sulphide from carbon dioxide byselective absorption in a liquid absorbent such as diethylbenzene,aniline or kerosene.

Practically all fuel gases, such as coke oven gas, Water gas,carburetted Water gas, and coal gas contain acid constituents such ashydrogen sul-` phide, carbon dioxide and cyanogen. These acidconstituents are generally scrubbed out of the gases with water or analkaline solution. The principal acid constituents which are removedfrom a coke oven gas when using an alkali carbonate absorbent solutionafter the constituents are stripped from the absorbent, consist of a gasmixture of about 80% H2S and 20% CO2. Such a gas is an important sourceof raw material wherein the H26 is used in the manufacture of chemicalcompounds. It is necessary, however, to separate the carbon dioxide fromthe hydrogen sulphide before the H25 can be used for making manycompounds.

I have found that the differences in solubility of H2S and CO2 in somespecial solvents is such that a substantially pure `hydrogen sulphidemay be recovered by absorption.

The primary object of the presentinvention is 1 Claim. (Cl. 23e-2) Afurther object of the invention is to provide a method of continuouslyseparating H25 in a pure state from gaseous CO2.`

A still further object is to provide a method o separating pure H2S fromCO2 in the gaseous state with an aniline absorbent.

With these and other objects in view, the in` vention consists in themethod `of separating H28 from CO2 in a gaseous state by selectiveliquid absorption as hereinafter described and` particularly defined inthe claim.

The various features of the invention are li1- lustrated in theaccompanying drawing which is a diagrammatic flow sheet of an apparatus:inV

an absorbent which is stable and selective. Furthermore, it is importantthat the separation of CO2 from H2S should be `complete and that thepurity of the H28 shall be Very high.

Many solvents have been tested and among the solvents tested which gavepromise of provid-v ing an absorbent for separating CO2 from H2S arethose solvents which are shown in the table below.

Tabla-Solubilities of H2S and CO2 in solvents Vol. Gas at 0 C. 760 mm.

Vol. Solvent at 26 C. Mis- Solvent VOL (.3102

Solubility H25 C02 Ratio l Equilibrium at 30 C. and l0 lbs.

Gage Pressure:

Tap Water 2. 1.24 2. 2 1.42 0.52 2. 7 7. 92 1.58 5.0 15. 0 3. 46 4. 3 9.74 2. 27 4. 3 4. 62 0.75 6.2 7. 2.03 8. 9 10.0 1. 96 5.1 p 12. s 3.16 4.o 8.54 2.43 3. 5 13. 3 2. 07 l 6.3 12.38 2.89 4. 3 22. S 1. 94 ll. 8 13.6 2. 8 4. 9

Typical examples of suitable absorbents are laniline, diethylbenzene andkerosene. When a mixture of CO2 and H255 with a major portion of H25 ispassed into any one of thepreferred absor` bentsolutions, all of the gasisabsorbed. The H2S is many times more soluble in the preferred sol- 55,vent than the CO2 and therefore as the Volume of gas increases, the H2Sacts to replace the CO2 and therefore the CO2 will be set free from theabsorbent solution as an exhaust gas. A continued absorption of HzS andCO2 in the preferred absorbent Will act to saturate the absorbent withHzS at which time the CO2 should all be driven out of the absorbent, if`the absorbent has the proper degree? of selectivity. Itv has` been foundthat aniline has a very high degree of selectivity and diethylbenzeneand kerosene will all produceY a hydrogen sulphide which has a 99.04-purity. Furthermore these solvents are capable or re*-l covering morethan 98% of the HzSsavailablein the acid gas mixture. In carrying outthe process, suicient H should be passed through the absorbent with theCO2 so that the absorbent becomes saturated with H28 and a small amountof H2S above thersatu-.v

rated condition of the absorbent will pass out withfthel CO2,y asexhaustgasz.; v

. I: have found' that carbon-dioxide may Vbe separated from hydrogensulphidebyv a continuous processjin which absorbent`r liquid passesdownwardlyV through a tower.Vv in countercurrentto` a` stream of gasmixture, passing upwardlyvthrough. a

theitower.- ,Due'tothe counter-current circulation of. -theabsorbentanrdgas,`itV is preferred I.to introducethe gasjmixture intothernidportionof the volumezof. absorbent. Further, when the absorb ent-is.saturated with H2S under a substantial pressure,4V the majorportion ofithe H25 may be separated-fromthe absorbent by merely releasing i thepressure onthe saturated absorbent. If, for example, the HzSr isabsorbed under five atmosl pheres of pressure',` thenwhen the, pressureis released on thesaturated absorbent to, reduce it.

toy atmospheric. pressure. four-fifths of the B2S willlbe separated from.the absorbent.v If it .-is.not important to maintain a high eiciencyrecovery of the H25, theny theabsorbent havingH2S therein may be reusedin the operation. TheY partially separated absorbent will thereby `havepart of its-"capacity to absorbcut Dfi.; Itis desirable that thev:saturated solutionY after the-pressure has been f relieved, shouldbedistilled with heat in order to removeall-of-the H2S. Bypressureabsorption a comparatively small amount of absorbent mayibe used forremoving a largevolume of H28 from agaseous mixture.

Referring to the drawings the preferredmethodM ofxthe invention may be'.carried out. as follows:`

A mixture *of HzS` and CO2.

This compressed gas passes through a. coolerl I4*Y into the bottom of anabsorption tower I. The

CO2 and H25 will be absorbed in theabsorbent inv the tower and then theCO2 will be displaced Vbytha-12S, the CO2 exhausting Athrough an. outletI8.. Theabsorbentisgintroducedinto the topof4 will. remain. :in i: theabsorbentv solution passing.

out :of'thef tower It through a :line 22: This ab-V is introduced:lthrough a linely I0 into a compressor I2. and.com..' pressed preferablyto about ve atmospheres:V

sorbent liquid enters the top of a rectifying absorber 24 and passesdownwardly therein in countercurent contact with I-IzS which isintroduced near the bottom of the absorber through a line 26. Ifdesired, the saturated solution of absorbent in the bottom'of the towermay be heatedby asteam;v coil 28; toinsurethat-all CO2 isdriven outofthe absorber. It isnot necessary to use both recycle H2S and steam toremove last `traces of CO2. Either will suice. The saturated solution ofabsorbent leaves the bottom of the tower 24 through a line 3B and passesinto the topf-of a stripping` column 32. Into the bottom of thestrippingcolumn 32 is introduced a stream of direct-steam: through a line 34. Thedirect contact of open steam with the absorbent heats the absorbentliquid' which assists in driving out the H25 andthe steam by its partialpressure assists in removing I-I2S from the absorber. The mixtureofsteam and H25 passes through a dephlegmator 36. wherein water isseparatedfrom,y tha-12S. gas..

and the H2S gas leavesthe dephlegmator throughY a line 38;.

The gas .passing out through the line, 3 8.- is pure. I-I2S vandthisstream is divided, one part. passing through a line 40 to a place ofstorage andthe.` other part, passingthrough a. line-.42. back` toA acompressor 44. Thestreamof pureH2S reaching the` compressor, ,iscompressed. .to. a pressure slightly higher than .thefpressure inthetower 1.4- andy is, then introduced into` the ,towerthrough the line`26.

The.. `absorbent passingv through the.. stripping..

column iiows through a .line 46 into a separator 48., andVcondensedsteam is-v withdrawn4 from; theT bottom of, the separator'.througha line.l 50. The.

absorbent liquidpasses throughraline. 52'. inftcf,

a collecting` tankY 54. .f From. theA tank .54' thefabf sorbentpassesthrougha vline 5,6 and is,fo'rced.by` Y desirable to introduce theabsorbent into the top of the tower I6 at a comparatively lowtemperature (50 to 100 F.) to maintain an eiicient absorption of theacid gases.

If the absorption towers and stripping column have suiiicient capacityfor handling the desired amountof gas and absorption solution, thecompressors I2 and 44 may be omitted and the gas canbe circulatedthrough the towers at substantially atmospheric pressure. When pressureis used in the absorption, reducing valve 'I0 is used inthe line 30 toreduce the pressure on the absorbent down to substantially atmosphericpressure:.before introducing the absorbent into the stripping column 32.This pressure reduction acts to strip out the H2S from the absorbent andassist the heat and steam in carrying out the stripping distillationoperation. I

In` the drawing` the absorptionand rectifying columns, are shownasseparate towers but.it .isa`. apparent'that one tower might be used inwhich;

the 2 absorbent; passes.. from thev top Atto' the; bottom when the rawgas-mixturefbeing ,introducedintorl the .mid portion, forv absorptionoperatiorrrandtthezf pure H2S being introduced near the bott'omof:the.;l

tower for stripping out CO2 entrained in the absorbent.

In carrying out the operation the amount of cool absorbent introducedinto the top of the tower I 6 should be such that practically all of theHzS will be absorbed and the absorbent will be saturated with HzSadjacent the outlet of the exhaust gas. In order to maintain a saturatedcondition of the absorbent, a small amount of I-IzS will always pass outof the exhaust line I8 with the CO2. `It is desirable to circulate theminimum amount of absorbent in carrying on the operation. Therefore thevolumes of gas and absorbent solution should be carefully measured tocarry out an eicient absorption.

As an example of the separation method, when using 60 lbs. pressure, agas mixture of HzS and CO2 composed of 22.3 mols of HzS and 5.6 mols ofCO2 per hour, or about 10,500 cu. ft. is passed through an absorptiontower into which is introduced 5400 gallons per hour of diethylbenzene-`From the stripping column substantially 22.3 mols per hour of pure I-IzSis removed through the line 40 while 27.9 mols or 10,500 cu. ft. ofI-IzS is circulated through the line 42 back to the compressor I4. Ifsubstantially the same volume of gas per hour is treated in theapparatus at atmospheric pressure, then 23,800 gallons per hour ofdiethylbenzene must be circulated down throughthe tower l0. From thedephlegmator of the stripping column 22.3 mols of pure I-IzS is removedthroughthe line 40 and 40.2 mols of pure HzS or 15,300 cu. ft. of HzS iscirculated through the line 42back to the rectifying column 24. 1780lbs. of open steam is used in rectifying the saturated absorbent in thestripping column for the pressure absorption operation whereas 1975 lbs.of open steam is required for stripping HzS out of the stripping columnwhen the absorption is carried out at atmospheric pressure. Theoperation outlined in the example given above provides a large excess ofHzS for removing the CO2 from the saturated absorbent. It has been foundin most cases that from 20% to 80% of the amount of HzS that was removedfrom the 4 line 40 is sufcient for removing all CO2 from the from thegroup consisting of kerosene and aniline,

controlling the relative volumes of liquid absorbent and gaseous mixtureto absorb substantialh7 all of the HzS from the gaseous mixture by thetime it reaches the top ofthe absorption zone, contacting liquidabsorbent from the absorption zone in a second multistage contact zonewith substantially pure HzS and thereby displacing CO2 from said liquidabsorbent and saturating it with I-IzS, and stripping HzS from thesaturated liquid absorbent in a stripping zone.

LESLIE W. ROYER.

l REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,422,183` Curme, Jr July 11,1922 1,725,925 Kent Aug. 27, 1929 `2,083,213 Baehr et al June 8, 19372,144,692 Schuftan Jan. 24, 1939 2,161,663 Baehr et al June 6, 19392,169,210 Balcar Aug. 15, 1939 2,180,386 Balear Nov. 21, 1939 2,264,878Hatch Dec. 2, 1941 OTHER REFERENCES Seidell: Solubilities of Inorganicand Organic Compounds, pp. 232 and 322, vol. I, 1919, Van Nostrand IncN. Y. l

